If you mean the customers of the IT service (who, in fact, are the collective customers of the whole of IT Services, rather than your customers), then you need to provide data that overall service management wants to fill out its picture of service quality.

If by customers you mean IT Service Management (and that is properly the "customer" of the helpdesk), then you probably want to know about your capability for first time fix, effective escalation, effective closure and the like.

Even putting semantics aside, it is not sensible to think of implementing ITIL for different areas as separate exercises. It would not be good for the Helpdesk Manager, the Change Manager, etc. to report independently on their areas as if to say "well my bit works".

The state of the service to customers needs to be evaluated holistically so that it can be managed and improved effectively and so that its business value can be understood._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718

When you ask a question that is not very specific or has much details, you will get sometimes silliness. Please look in the miscellaneous forum for the Sixth Book and other posts

When you ask for stats about your section of the help desk, what do you mean ?

Are you and your team the day shift and you want to compare against the evening or weekend shift
Are you the 2nd & 3rd line support for incidents >
Are you the regional / local Helpdesk and you want to compare against the global / national helpdesk

Your initial question and my silly response matches quite well. Both are completely worthless by them selves yet they do answer something

A key phrase in your post was - diagnose the state of my customer service.

Diarmid spoke to that

The answer is rather difficult ?

If you use merely numericial statistics, you will get one view
If you have customer surveys, you will get another view
If you have SLAs, OLAs and such like and you develop a baseline about the meeting of the SLAs, OLAs, KPIs, then you will get another view
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In other words, there is no real answer.

If I say - 80% of your calls should be closed in X minutes/hours /days -

that would only work if your organization works exactly the same type of calls that the statistics came from

If your organization (Helpdesk) spends 30 - 40 minutes with a customer to diagnose / solve their problem on the phone, and you get 10 - 12 calls a day (weekday), you would have a different set of statistic from a company that spends 3 - 4 minutes with a customer and 100 - 200 calls a day.

And like I said.... chipmunks are the best Service desk types.. they will work for 'peanuts' (low wages)_________________John Hardesty
ITSM Manager's Certificate (Red Badge)